1. Field of the Invention
The present invention relates to a structure of a II-VI semiconductor laser device which has a stable lateral mode and is capable of oscillating with a low threshold current, and a method for fabricating the same.
2. Description of the Related Art
Semiconductor laser devices emitting blue or blue-green light using the double heterostructures consisting of II-VI compound semiconductor have been proposed. First of all, semiconductor laser devices having double heterostructures, in which ZnMgSSe or ZnSSe formed on a GaAs substrate serves as a cladding layer, have already been realized. Such semiconductor laser devices has been reported in "Electronics Letters", Vol. 29, No. 9 (1993), page. 766 or "The Journal of the Institute of Electronics, Information and Communication Engineers", July 1993, page 818. However, these devices have a problem that a current blocking function and light confinement in the direction parallel to the junction plane cannot be sufficient since the devices have stripe contact structures.
Therefore, "Electronics Letters", Vol. 29, No. 23, (1993) pp. 2041-2042, has reported the improvement of the disadvantage of the stripe contact structure: a resin buried ridge waveguide type semiconductor laser device structure adopted in a III-V semiconductor laser device of AlGaAs or the like (Japanese Laid-Open Patent Publication No. 63-122187) is applied to the abovementioned II-VI compound semiconductor. FIG. 4 is a cross-sectional view showing the buried ridge waveguide type semiconductor laser device. An n-GaAs substrate 21 is used as a substrate. A buffer layer 22, an n-MgZnSSe lower cladding layer 23, an n-ZnSSe layer 24, a CdZnSe quantum well active layer 25, a p-ZnSSe layer 26, a p-MgZnSSe upper cladding layer 27, a p-Zn(Se, Te) contact layer 28 are successively deposited on the n-GaAs substrate 21. On the both sides of a striped ridge 27a which is formed by partially etching the upper cladding layer 27 and the contact layer 28, a burying blocking layer 29 made of polyimide resin is deposited. Electrodes 2a and 20 are provided on the upper surface of the striped ridge 27a and the lower surface of the substrate 21, respectively. Although the electrode 2a is formed on the upper surface of the striped ridge 27a alone in FIG. 4, the electrode may be formed on the entire surface of the device. Even if the electrode is formed on the entire surface of the device, the device characteristics are not affected thereby.
A current flowing between the upper and the lower electrodes 2a and 20 is injected to a portion of the active layer under the striped ridge 27a alone owing to blocking effect of the polyimide resin burying blocking layer 29. Furthermore, since the upper cladding layer 27 other than the striped ridge 27a does not have a sufficient thickness to confine light therein, the light propagates through the striped ridge 27a alone. In this way, the buried ridge waveguide type semiconductor laser having the structure shown in FIG. 4 has both the current blocking mechanism and the optical waveguide mechanism.
In the ridge waveguide type semiconductor laser device shown in FIG. 4, however, satisfactory device characteristics cannot be obtained. Specifically, some of the problems such as poor mounting of devices, deterioration of a lifetime of devices, degradation of threshold value current characteristics and poor operation at high output occur. Thus, the conditions which are capable of solving all these problems could not be found.